1. Technical Art
The present invention pertains to hydraulic cylinders in general, and to solving seal stiction in hydraulic cylinders in particular.
2. Background Information
Hydraulic cylinders generally comprise a piston displaceably mounted within a housing. The housing includes at least one port through which fluid can be passed into or out of the housing. Fluid pressing against the surface area of the piston perpendicular to the stroke of the piston will force the piston out of the housing when the pressure of the fluid is great enough to overcome the load supported by the piston. Likewise, when the pressure of the fluid provides insufficient force to overcome the load supported by the piston, the piston will retract within the housing. Physical stops within the housing prevent the piston from retracting completely into, or extending completely out of, the housing. One or more seals around the piston prevent pressurized fluid from leaking out between the housing and the piston, thereby maintaining the pressure difference between the fluid inside the housing and the environment outside.
A person of skill in the art will recognize that the seals preventing fluid from escaping around the piston influence the amount of force necessary to move the piston. When the piston is moving, the seals provide an essentially constant amount of resistive force to the movement of the piston. When the piston is stationary, however, the amount of resistive force provided by the seals changes as a function of time. Immediately after the cylinder piston stops, the seal in contact with the piston begins to conform with the sealing surface of the piston, thereby increasing the surface area in contact between the seal and the piston. As the contact area increases, the amount of resistive force increases, and therefore the amount of force necessary to move the piston as well. After a short period of time, the percentage of surface area in contact between the piston and the seal does not appreciably increase. Thereafter, the force necessary to move the piston does not appreciably change either.
The difference in force necessary to move the piston between the static state and the dynamic state causes a motion control problem known in the art as "stiction". The control problem occurs when the force necessary to move a static piston remains applied after the piston has begun moving, thereby causing the piston to accelerate. The acceleration causes the piston to jerk, somewhat erratically. By way of an example, a hydraulic elevator having an elevator car constrained to motion within a hoistway is powered by a hydraulic cylinder. If stiction is present, a passenger in the elevator will experience a jarring sensation as the car goes from a static state at a landing to a dynamic state leaving the landing. Any abrupt fluctuation in elevator motion is perceived as an indicium of poor quality. Hence, it is desirable to minimize or eliminate stiction in a fluid cylinder.